1. Field of the Invention
This invention relates to a fuel burning heater, and more particularly to a fuel burning heater having a controlled burning stability in which a stable burning condition is maintained regardless of occasional changes of heating power.
2. Related Art
A typical fuel burning heater is equipped with a burner which diverts a portion of the fuel to be supplied to a combustion engine to the heater and burns the fuel to increase the temperature of water circulated in the heater. The heated water passes into a heat radiating unit, and the heated water exchanges heat with air introduced in the heat radiation unit, thereby supplying warm temperature air into a passenger compartment. This kind of fuel burning heater is an auxiliary heating device used for a short and limited duration from a cold start-up of the engine until the engine is warmed up.
More specifically, the burner of such a fuel burning heater vaporizes the fuel supplied therein, mixes up the vaporized fuel with an air introduced therein, and burns the mixture of vaporized fuel and air by means of an appropriate igniter such as a glow plug.
FIG. 5 is a cross-sectional view showing an overall arrangement of a conventional fuel burning heater. In the drawing, a housing 1 is formed with a fluid passage 13 defined in the wall thereof, and accommodates a burner 2 in the inside space surrounded by the wall. The burner E has a mixing cylinder 21 having a closed distal end equipped with a plate-like vaporization member 81 made of ceramic fiber, the vaporization member 81 extending across the mixing cylinder 21. The vaporization member 81 is brought into contact with one end of a fuel supply pipe 33. The other end of the fuel supply pipe 33 is connected to a fuel pump 32 provided in a fuel tank 31. Thus, fuel in the fuel tank 31 is sucked up by the fuel pump 32 and introduced into the vaporization member 81. The fuel supplied into the vaporization member 81 spreads or diffuses across the entire body thereof, aided by the capillary phenomenon. A glow plug 82 is located adjacently to the vaporization member 81, so as to heat the vaporization member 81. Thus, the vaporized fuel is generated from the vaporization member 1 and then mixed with air in an inside space of the mixing cylinder 21, the air being introduced through air inlet holes 211.
More specifically, the air flows into the inside space of housing 1 from an inlet port 14 formed on an upper wall of the housing 1, the inlet port 14 communicating with an air supply pipe 41. The introduced air enters into the mixing cylinder 21 and is mixed with the vaporized fuel of the vaporization member 81, and then thus formed gas mixture of fuel and air is fired in a burning cylinder 22. Exhaust gas, generated in the burner 2 through combustion of the gas mixture, is discharged or scavenged from an exhaust port 15 formed on the lower wall of the housing 1, the exhaust gas flowing along an exhaust pipe 81 and going out of this fuel burning heater system.
An air pump 42 is provided in the air supply pipe 41 to adjust the air amount to be supplied into the burner 2. As well as the fuel pump 32, the air pump 42 is controlled by a control unit 7 so as to obtain a desired heating power.
This kind of fuel burning heater is, for example, disclosed in the unexamined Japanese patent application No. 1-262214/1989, the unexamined Japanese patent application No. 4-73503/1992, the unexamined Japanese patent application No. 4-214105/1992, or the U.S. Pat. No. 4,538,985.
In operation, the heating power of the above described conventional fuel burning heater is generally reduced with increasing temperature of the passenger compartment or the like. To realize such an adjustment, the fuel supply amount of the fuel pump 32 is reduced as shown in FIG. 6A, while the air supply amount of the air pump 42 is correspondingly reduced as shown in FIG. 6B, thereby maintaining an air-fuel ratio of the gas mixture at an appropriate value.
However, simply reducing air supply amount to be entered into the burner 2 causes reduction in the air flow speed and pressure, which correspondingly induces variations in a burning pressure of the burner 2 and a ram pressure upstream of the air pump 42, with possibly variation of an air supply amount to be supplied into the burner 2 due to the influences thus caused.
To solve this problem, it may be possible to adjust the heating power of the burner by the ON-OFF control of the burner which periodically repeats an ignition of the burner and an extinction of the same. However, such an ON-OFF control of the burner is not desirable in that harmful emission of HC (hydro-carbon) increases due to incomplete combustion of the air-fuel gas mixture derived from the intermittent firing of the burner.